System for a two-stroke crankcase scavenged combustion engine

ABSTRACT

The present invention relates to a system ( 1 ) for supplying an air-fuel mixture to a two-stroke internal combustion engine cylinder ( 2 ), comprising a carburettor ( 3 ) having a throttle valve ( 4 ) and usually a choke valve. The system further has a transfer passage ( 6 ) between a crankcase chamber and a combustion chamber of said engine. A supply conduit for additional air ( 8 ) is having an air valve ( 10 ) for said additional air, and said supply conduit for additional air ( 8 ) is being adapted to discharge the additional air into the top portion of said transfer passage ( 6 ) during a portion of a cycle of said two-stroke internal combustion engine. Further the air valve ( 10 ) is controlled by the throttle valve ( 4 ) so as to affecting a air valve opening state when said throttle valve opening state is affected. Said air valve ( 10 ) is of a barrel valve type while said throttle valve ( 4 ) is of a butterfly valve type.

TECHNICAL FIELD

The present invention relates to a system for a two-stroke crankcasescavenged internal combustion engine, in which an air passage foradditional air is arranged between an air inlet and the upper part of atransfer duct. Fresh air is added at the top of the transfer duct and isintended to serve as a buffer against the air/fuel mixture below. Thisbuffer is mainly lost out into the exhaust outlet during the scavengingprocess. The fuel consumption and the exhaust emissions is therebyreduced.

BACKGROUND OF THE INVENTION

For conventional two-stroke combustion engines it is well known how tocontrol the air/fuel mixture, but a high level of unburned hydrocarbonemissions is produced because in this engine the scavenging process isperformed exclusively by a mixture of air and fuel. Some of the air/fuelmixture mixes with the exhaust gases and thereby some unburned fuel islost out with the exhaust gases.

Two-stroke combustion engines using the additional air technique duringthe scavenging are previously known in the art. These engines reducefuel consumption and exhaust emissions. But for these prior art enginesit is a problem to control the air/fuel mix during normal operation andduring start. One common start procedure for a conventional two-strokeengine is with a closed choke valve and a fully open throttle valve. Incombination with a “conventional” additional air technique this willprovide too much air for optimal starting conditions. Since these kindof engines are used in many different environments and subjected totroublesome conditions it is desired to achieve a reliable design with arobust operation. One issue with engines using the additional airtechnique is to also provide good start properties. The operationalconditions for this kind of engines can be of varying temperature,humidity, atmospheric pressure etc. The manufacturer Walbro has launcheda carburettor comprising a double barrel valve with one part for themixture and one part for the additional air. This however is a rathercomplicated design and much more expensive than a conventionalcarburettor. U.S. Pat. No. 6,328,288 shows a system comprising twobutterfly valves. An elongated slot is arranged in the throttle valvelever for fastening the link to the air valve lever to give a delayedopening of the air valve. This arrangement of a link is not verysatisfactory as there is an increased risk that the link will fallapart. Further there is a risk of ice build-up in the slot in thewinter. This will make starting very difficult.

Hence, there is a need for an improved system for a two-stroke engineusing the additional air technique to achieve good properties for startand normal operation. Further it is an advantage to combine properoperation of such an engine with lean and environmentally friendlyoperation. Finally it would be advantageous to provide a robust, costeffective and high-quality system for this kind of engine.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a system for supplyingan air/fuel mixture to a two-stroke internal combustion engine thatovercomes at least some of the above-mentioned matters and present animproved control of the air/fuel mixture to the engine.

According to the invention there is provided a system for supplying anair-fuel mixture to a two-stroke crankcase scavenged internal combustionengine, comprising a carburettor having a throttle valve.

The system further has a transfer passage between a crankcase chamberand a combustion chamber of the engine, and a supply conduit foradditional air having an air valve for said additional air. The supplyconduit for additional air being adapted to discharge the additional airinto the top portion of said transfer passage, either directly through acheck valve or via a recess in an engine piston, during a portion of acycle of said two-stroke internal combustion engine. The air valve iscontrolled by said throttle valve so as to affecting a air valve openingstate when said throttle valve opening state is affected, and in thatsaid air valve is of a barrel type while said throttle valve is of abutterfly valve type.

In prior art two-stroke engines with additional air there is a problemwith too much additional air through the supply conduit when the chokevalve in the carburettor is closed. The present invention provides animproved solution for two-stroke engines using additional air. With thissolution it is possible to control the air/fuel mixture during start-up,normal operation and other conditions for the engine.

Advantageously the air valve is resiliently connected to said throttlevalve so as to affecting a air valve opening state when the throttlevalve opening state is affected and the choke valve in a choked state isadapted to delimit the opening state of said air valve regardless of thethrottle valve opening state. This resiliently connection between theair valve and the throttle valve can be performed in different ways, forexample by levers connected by a spring, spring loaded rod with flexiblelength, telescopic rod, oval holes in combination with springs or leverswith interacting teeth. In an alternative design of the system a controldevice for controlling the supply of additional air due to the chokeopening state can be provided adjacent to the air valve, with influencefrom the choke valve.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, embodiments of the present invention will now bedescribed with reference to the accompanying drawing figures in which:

FIG. 1 is a perspective side-view showing a two-stroke engine cylinderprovided with a system for supplying air-fuel mixture and for supplyingadditional air during the scavenging process.

FIG. 2 shows the same system but with the main parts drawn away fromeach other for clarity reasons.

FIG. 3 a-3 d show in a schematic series how a throttle valve and an airvalve of the invention works together.

FIG. 4 a-4 c is a side-view of the air valve with its barrel in threedifferent positions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment of the present invention will be described in moredetail in the following, with reference to the accompanying drawings.

Referring to FIGS. 1 and 2, a two-stroke engine cylinder 2 withcarburettor 3 and a system for additional air 9 is shown. Thecarburettor 3 is provided at an intake of the cylinder. Further is thepiston provided for letting air/fuel mix into the crankcase and freshair, for the scavenging process, into the upper part of a transferpassage 6 during a portion of an engine cycle. During a stage of theengine cycle the air/fuel mixture is compressed in the crankcase andthereafter the exhaust gases is pressed out through the exhaust port bythe compressed air/fuel mixture. The buffer of fresh air in the transferpassage 6, provided from the additional air system, enter the combustionchamber before the air/fuel mixture. Consequently the risk foruncombusted fuel to accompany the exhaust gases out from the combustionchamber is reduced. The additional air 9 is provided into the transferpassage 6 through a channel 8 and an aperture in the cylinder 24. Whichaperture, during a portion of the engine cycle, leads into a recess inthe piston fluidly connected to the transfer passage 6. The channel 8for additional air could also connect directly to a transfer passage 6via a check valve or so called Reed valve. The flow of additional air 9into the transfer passage 6 is substantially simultaneous with the flowof air/fuel mixture into the crankcase.

The additional air channel 8 is provided with an air valve 10 forcontrolling the additional air to the engine 2. The air valve 10 foradditional air is interconnected with the throttle valve 4 of thecarburettor 3, this is to give synchronous control of the opening stateof the throttle and the supply of additional air via the air valve 10.

In more detail this is arranged as shown in FIGS. 1 and 2. To the leftthere is a filter housing 13 shown without a lid and a filter element.It has two ducts, an upper duct for supplying additional air 9, and alower duct for supplying air to the carburettor 7. As seen in FIG. 2 thefilter housing 13 has an air intake 32, which supplies air at the bottomof the filter housing. From the bottom of the housing rise a number ofridges 33 and an upper tube 35 and a lower tube 34. The filter elementrests on top of the ridges 33 but is provided with holes for the upperand lower tube. Therefore air from the air intake 32 will go from thebottom through the filter element and will from there be sucked into theintake for additional air and the intake for air to the carburettor. Theadditional air 9 flows to an air valve 10 that is of a barrel valvetype. Said air valve comprises an air valve housing 20 which forms anintegrated part of the filter housing 13. This is a cost effectivedesign which also prevents leaks between a separate valve and the filterhousing 13.

A shielding baffle 37 is fastened to the cylinder and crankcase by anumber of screws fastened through the apertures 38, 39. The two lowerapertures 38 are made for screws to be fastened in the crankcase, notshown. The upper aperture 49 holds a nose 36 on the filter housing 13.The screw is fastened through an aperture in the nose 36 and into theaperture 39 (secured in the cylinder). The shielding baffle 37 shieldsthe fuel system from a strong heat from the engine cylinder 2. Itfurther has a protruding part 40, which ends in a fastening plane 41.The plane has two fastening holes 42 and a sealing 43 that seals betweenthe plane 41 and the carburettor 3. When mounting the system acarburettor 3 is located correctly on the fastening pane 41 and thefilter housing 13 is placed on the carburettor and two screws aresecured through the filter housing. The holes that are not visible runthrough holes 48 in the carburettor and are secured in holes 42 in theshielding baffle 37. In the protruding part 40 there is a duct 49 thatleads the mixture from the carburettor into the intake port of thecylinder. The shielding baffle further has two short ducts 48, as can beseen in FIG. 1. The branched rubber fitting 44 is fitted to the twotubes 48 in the shielding baffle and to the outlet part 45 of air valve10. As shown in figure the air valve 10 holds a number of parts. Firstthe sleeve 46 is inserted in the opening of the air valve and then thebarrel cylinder 15 is inserted inside the sleeve. The barrel cylinder 15has a shaft 47. An end plate 50 is placed over the shaft and screwed tothe air valve. The end plate 50 is arranged to house a spring that isattached to the air valve lever 23 that is attached to the shaft 47 andtherefore rotates the barrel 15 and opens and closes the valve.

FIG. 3 a-3 b schematically show how this system works, i.e. how thethrottle valve 4 and the air valve 10 work together. In FIG. 3 a boththe throttle valve and the air valve 10 are fully closed. You can seethat the throttle valve 4 and the rotatable barrel 15 both are in fullyclosed positions. As explained earlier, the two valves areinterconnected by a rod or link 24. In this series of figures thisinterconnection is indicated by a movable rod 51 to the left of the twovalves. In FIG. 3 b both the throttle valve 4 and the barrel 15 havebeen rotated by rotating the throttle valve itself. The position in thisfigure shows an open throttle valve 4 and a closed barrel valve 10, butjust about to be opened. In the next FIG. 3 c the two valves have beenrotated a bit further and are both approximately half open. Thereforethe additional air 9 can pass the barrel valve in FIG. 3 c, while thiswas not possible in the two earlier FIGS. 3 a, 3 b. In FIG. 3 d bothvalves are fully opened, i.e. they have both been rotated approximately90 degrees. Therefore they don't throttle the flow of additional air 9or air flow through the carburettor 7. As explained in connection toFIG. 1 the supply conduit 8 for additional air 9 has a first end 12 in afilter housing 13 and a second end in an additional air port in acylinder wall. However, between the two ends there is a fixed flowrestriction 14, as evident in FIGS. 3 and 4. The fixed flow restrictionis located between the first end 12 and the rotatable barrel 15 in thebarrel valve 10. As evident in FIGS. 3 and 4 the fixed restriction 14covers a part of the length of a first conduit part 17 between the firstend 12 and the rotatable barrel 15, and ends at the rotatable barrelrestricting an intake opening 18 to the barrel. This fixed restrictiontherefore starts at the rotatable barrel and covers a part of the lengthtowards the first end 12. In FIG. 4 it covers the whole length, but thisnot necessary.

The fixed restriction has a number of advantages. First it throttles theadditional air flow 9 even when the rotatable barrel 15 is fully openedas shown in FIGS. 3 d and 4 c. This throttling reduces the amount ofadditional air at high engine speeds, which results in an improvedair-fuel mixture ratio. Without such a fixed restriction the air-fuelmixture will be too lean in these speeds. Further this restrictioninfluences the angle which the rotatable barrel 15 can be rotated beforeopening. This angle can be seen by comparing FIGS. 3 b and 3 a as wellas by comparing FIGS. 4 b and 4 a. This is of course provided that thefixed restriction meets the rotatable barrel, which is a preferredembodiment. If instead the first conduit part 17 would have had the samediameter as the second conduit part 5 the angle of rotation for openingthe valve of the barrel 15 would be smaller. It is an advantage to havea relatively large angle for the barrel to rotate before it starts toopen the air valve. When starting the engine it is important that theair valve is closed and a late opening of the air valve promotesacceleration of the engine.

The fixed restriction can also be located in the rotatable barrelitself. Preferably the restriction covers the full length of theaperture in the barrel. The restriction could also be located between adownstream end of the barrel itself restricting the outlet opening 19from the barrel 15 and the downstream end of the barrel valve. It ispreferable to have the restriction in this location so it affects theaperture through the air valve. Preferably the restriction has an areaof more than 0,1× the area of the unrestricted conduit area and of lessthan 0,7× the unrestricted area and preferably of more than 0,2× theunrestricted conduit area, and of less than 0,5× the unrestrictedconduit area. The length of the restriction is not of very bigimportance. Therefore it can be adjusted according to economy ofmanufacturing etc. The fixed restriction 14 could also be locatedfurther downstream in the supply conduit 8, e.g. in the apertures in theshielding baffle 37, or even in the ports of the engine cylinder. Thefixed restriction could also be used if air valve is instead of abutterfly valve type though this is less preferable.

As evident by this discussion there is a distinct advantage in choosingan air valve 10 of a barrel valve type instead of a butterfly valvetype. The barrel can be rotated a considerable angle before opening,while the butterfly valve starts to open immediately. By choosing abutterfly valve type for the throttle valve and a barrel valve type forthe air valve it is therefore possible to have preferablecharacteristics concerning opening of the two valves. And further, theangle of rotation of the barrel before opening can easily be adjusted bychanging the size of the inlet or outlet or even the size of theaperture of the barrel valve itself, as shown above. This combinationfurther has the advantage that a standard carburettor can be used, andfurther it is possible to use a rod or link attached to the air valvelever and to the other lever without having an elongated slot in eitherlever. This gives a more reliable control system.

FIG. 4 a-4 c show the air valve 10 of barrel valve type and how the airvalve housing 20 forms an integrated part of the filter housing 13, asdescribed before.

As shown in FIGS. 1 and 2 the carburettor 3 can also be provided with achoke valve 29. In FIG. 2 a shaft for the choke valve is visible but notthe valve plate itself that should be fastened to the shaft. This is forclarity reasons. It is preferable to provide the carburettor with achoke valve 29 upstream of the throttle valve and provided with a chokevalve lever 30. This lever 30 is also provided with a cam 31 forinteraction with the spring actuated lever 25 when choke valve 29 ismoved from an open state. Hereby the cam 31 locks the spring-actuatedlever 25 so that it cannot rotate and open the air valve 10 throughrotating the air valve lever 23 via the rod 24. Instead when thethrottle valve is actuated, this only tensions a spiral spring 26attached between the throttle valve lever 22 and the spring actuatedlever 25. So, the throttle valve lever 22 is interconnected with the airvalve lever 23 via a spring and the spring-actuated lever 25 is movingthe air valve lever via the rod 24. This arrangement makes it possibleto start with a full choke setting combined with many different throttlesettings up to full throttle without opening the air valve. For manykind of handheld tools, e.g. trimmers, it is a common and preferred wayto start the tool using full choke combined with full throttle. In manyother tools, e.g. chain saws, it is however more common to start thetool using full choke and only a limited degree of throttle. For such atool it is not necessary to use a spring actuated lever 25 rotated by aspring 26. Instead it is possible to use only a throttle valve lever 22that directly holds a rod 24 that turns the air valve lever 23.

When using a system that enables full throttle/full choke start, e.g. asystem having a spring actuated lever 25, it is preferable that a spiralspring 26 is attached to said throttle valve lever, and acting on thespring actuated lever 25, pushing a first stop 27 on the spring actuatedlever against a second stop 28 on the throttle valve lever 22. Herebythe spiral spring 26 is pre-tensioned which gives a more accurate workof the spring-actuated lever maneuvering the air valve lever. The systemis especially preferable for handheld working tools provided with atwo-stroke crankcase scavenged internal combustion engine.

1. A system (1) for supplying an air-fuel mixture to a cylinder (2) of atwo-stroke crankcase scavenged internal combustion engine, comprising acarburettor (3) having a throttle valve (4) and feeding an air-fuelmixture to an intake port of the cylinder, said system further has atransfer passage (6) between a crankcase chamber and a combustionchamber of the engine, and a supply conduit (8) for additional air (9)having an air valve (10) for the additional air, and said supply conduitfor additional air (9) being adapted to discharge the additional airinto the top portion of the transfer passage (6), either directlythrough a check-valve or via a recess in an engine piston, during aportion of a cycle of said two-stroke internal combustion engine,characterised in that said air valve (10) is controlled by said throttlevalve (4) so as to affecting an air valve opening state when saidthrottle valve opening state is affected, and in that said air valve(10) is of a barrel valve type while said throttle valve (4) is of abutterfly valve type (4), wherein a shaft (21) of said throttle valve isprovided with a throttle valve lever (22), which lever is interconnectedwith an air valve lever (23) via a spring and a spring actuated lever(25) moving the air valve lever via a rod (24).
 2. A system (1)according to claim 1, wherein in said supply conduit (8) for additionalair (9) has a first end (12) in a filter housing (13) and a second endin an additional air port in a cylinder wall, and between the two endsthere is a fixed flow restriction (14).
 3. A system (1) according toclaim 2, wherein said fixed flow restriction (14) is located between thefirst end (12) and a rotatable barrel (15) in the barrel valve (10). 4.A system (1) according to claim 3, wherein said fixed restriction (14)covers a part of the length of a first conduit part (17) between thefirst end (12) and the rotatable barrel (15), and ends at the rotatablebarrel restricting an intake opening (18) to the barrel (15).
 5. Asystem (1) according to claim 1, wherein said air valve (10) comprisesan air valve housing (20), which forms an integrated part of the filterhousing (13).
 6. A system (1) according to claim 1, wherein the springis a spiral spring (26) attached to said throttle valve lever and actingon the spring actuated lever (25) pushing a first stop (27) on thespring actuated lever against a second stop (28) on the throttle valvelever (22).
 7. A system (1) according to claim 1, wherein thecarburettor (3) is provided with a choke valve (29) upstream of thethrottle valve and provided with a choke valve lever (30).
 8. A system(1) according to any one of the claims 1 and 6-7, wherein at least oneof said spring actuated lever (25) and said choke lever (30) is providedwith a cam (31) for interaction, when said choke valve (29) is movedfrom an open state.
 9. A two-stroke internal combustion engine providedwith a system (1) for supplying an air-fuel mixture according toclaim
 1. 10. A hand-held working tool provided with a two-strokeinternal combustion engine having a system (1) for supplying an air-fuelmixture according to claim
 1. 11. A system (1) for supplying an air-fuelmixture to a cylinder (2) of a two-stroke crankcase scavenged internalcombustion engine, comprising a carburettor (3) having a throttle valve(4) and feeding an air-fuel mixture to an intake port of the cylinder,said system further has a transfer passage (6) between a crankcasechamber and a combustion chamber of the engine, and a supply conduit (8)for additional air (9) having an air valve (10) for the additional air,and said supply conduit for additional air (9) being adapted todischarge the additional air into the top portion of the transferpassage (6), either directly through a check-valve or via a recess in anengine piston, during a portion of a cycle of said two-stroke internalcombustion engine, characterised in that the supply conduit (8)comprises three parts; a first conduit part (17) leading from a firstend (12) in a filter housing (13) and to a rotatable barrel (15) in therotatable barrel valve (10), the rotatable barrel forming a barrelconduit part being followed by a downstream conduit part leading to asecond end in an additional port in a cylinder wall and there is a fixedflow restriction (14) in at least one but not in all the three conduitparts, and said air valve (10) is controlled by said throttle valve (4)so as to affecting an air valve opening state when said throttle valveopening state is affected, and in that said air valve (10) is of abarrel valve type while said throttle valve (4) is of a butterfly valvetype (4).
 12. A system (1) according to claim 11, wherein said fixedflow restriction (14) is located between the first end (12) and arotatable barrel (15) in the barrel valve (10).
 13. A system (1)according to claim 12, wherein said fixed restriction (14) covers a partof the length of a first conduit part (17) between the first end (12)and the rotatable barrel (15), and ends at the rotatable barrelrestricting an intake opening (18) to the barrel (15).
 14. A system (1)according to claim 11, wherein the fixed restriction (14) is located inthe rotatable barrel and preferably covers its full length.
 15. A system(1) according to claim 11, wherein said air valve (10) comprises an airvalve housing (20), which forms an integrated part of the filter housing(13).
 16. A system (1) according to claim 11, wherein a shaft (21) ofsaid throttle valve is provided with a throttle valve lever (22), whichlever is connected directly with an air valve lever (23), provided on ashaft of said air valve, via a rod (24).
 17. A two-stroke internalcombustion engine provided with a system (1) for supplying an air-fuelmixture according to claim
 11. 18. A hand-held working tool providedwith a two-stroke internal combustion engine having a system (1) forsupplying an air-fuel mixture according to claim 11.